1. Technical Field
The present invention relates to a liquid crystal display device having a transmissive display portion configured to perform a transmissive display operation and a reflective display portion configured to perform a reflective display operation in a pixel forming a display portion, wherein the transmissive display portion and the reflective display portion have a first substrate including a first electrode and a second electrode configured to control an alignment of liquid crystal molecules of a liquid crystal layer included inside the first and second electrodes.
2. Related Art
High visibility is required in various environments from outdoor to indoor for liquid crystal display devices to be used in mobile devices represented by personal digital assistants (PDAs), digital still cameras and digital video cameras. In recent years, in such a background, liquid crystal display devices of the so-called semi-transmissive type have attention, these devices having both a transmissive display portion and a reflective display portion in a sub-pixel.
Semi-transmissive liquid crystal display devices that have been generally used so far adopt a so-called vertical electric field driving method by which an electric field between a pixel electrode of an element substrate and an opposite electrode of an opposite substrate drives liquid crystal molecules. In a semi-transmissive liquid crystal display device having a transmissive display portion and a reflective display portion in a sub-pixel, a light path in the reflective display portion is twice as long as a light path in the transmissive display portion. Therefore, it is necessary to use half wavelength (λ/2) light modulation and quarter wavelength (λ/4) light modulation in a transmissive display mode and in a reflective display mode, respectively, which may be satisfied by different thicknesses (a cell gap) of the liquid crystal layer in the sub-pixel.
Higher visibility than usual may be expected by using semi-transmissive liquid crystal display devices adopting a so-called horizontal electric field driving method such as FFS (Fringe Field Switching) and IPS (In Plane Switching) which are well known for having wide view angles and high contrast. A problem has been pointed out, however, that the transmissive display portion and the reflective display portion are made normally black and normally white, respectively, by using the known configuration to arrange the cell gap and applying the horizontal electric field driving method.
For example, JP A-2005-338256 mentions that the reflective display portion is in a bright display state and the transmissive display portion is in a dark display state, or vice versa, if the semi-transmissive IPS method is simply used to apply a horizontal electric field to the liquid crystal layer. It is pointed out, according to JP A-2005-338256, that the reflective display portion and the transmissive display portion show dependency on applied voltages different from each other. It is disclosed, in order to solve this problem, that a built-in phase plate of retardation of a half wavelength is formed in the reflective display portion and the retardation of the liquid crystal layer of the reflective display portion is arranged to be a quarter wavelength.
It is pointed out, according to JP A-2003-344837, that a reflective region and a transmissive region are in normally white and normally black states, respectively, if a semi-transmissive liquid crystal display device is driven by a horizontal electric field. It is disclosed, in order to solve this problem, that a plate of a half wavelength is arranged between a lower substrate and a polarizing plate arranged on a side of the lower substrate.
As described above, the semi-transmissive liquid crystal display device of the so called horizontal electric field driving method has the problem that the transmissive display portion and the reflective display portion are in the normally black and normally white states, respectively. The methods of JP A2005-338256 and JP A-2003-344837 may be used in order to restore normal display states, i.e., to have both of the transmissive display portion and the reflective display portion in the normally black or normally white states. In that case, however, there are problems that the reflective display operation lacks sufficient reflectance and so on, although the transmissive display operation exhibits sufficient display performance, and that a configuration of a display panel is made complicated. In contrast, different methods of driving liquid crystal molecules may be separately applied to the transmissive display portion and the reflective display portion so as to have both of the transmissive display portion and the reflective display portion in the normally black or normally white state. In that case, however, there is a problem that the transmissive display portion suffers from light leakage around an interface to the reflective display portion and resultant low contrast.